The present invention relates to a milling cutter.
More particularly, the invention provides new forms for end mills cores, to produce a tool of improved stiffness having more versatile applications.
Milling cutters are rotatable tools of cylindrical, conical, shaped or disk form, having a plurality of cutting edges. Such cutters are available in many forms, such as plain cylindrical, side milling cutters, face and end mills, formed cutters, and standard and special shaped profile cutters. High speed steel cutters are used for short production runs, inserted carbide blades are often used for long runs.
The end mill is a simple cutter, often of cylindrical shape, wherein one end of the circumferential surface has spiral cutting teeth and the other end is blank to serve as a shank, except if the cutter is double ended, in which case the central portion is gripped by the machine. Teeth are also provided at the cylinder end face. Four flute mills are probably the most common, but 2, 3 or 6 flutes are also used extensively. End mills are in much used because they can execute a wide variety of milling operations, and the first cost of the cutter is moderate. Shapes other than cylindrical are also in common use. The shank can be parallel or tapered, and need not necessarily be equal to the cutter teeth diameter.
Much research has been carried out to determine the largest possible volume of metal removed before tool failure, in relation to a chosen cutting speed. There are however so many factors involved, such as workpiece machinability rating, which itself is a function of both material type and heat treatment, tooth form, cutter size, number of teeth and cutter material, machine tool power available at the cutter and machine tool rigidity, cutter rigidity, coolant type and flow rate, surface finish required, feed rate chosen, and depth and width of cut that results published for one application are difficult to relate to other applications even where the basic type of work, e.g. face milling, is the same.
Recent examples of attempted improvements in milling cutters, particularly end mills, are found in U.S. Pat. No. 5,779,399 to Kuberski and U.S. Pat. No. 5,810,517 to Bostic. These documents reference many earlier designs.
In the design of an end mill, the total diameter must be allocated between the tooth depth on both sides of the core and the remaining core diameter. A large core diameter will provide the advantage of a rigid tool. However, extended tooth depth will encourage good chip clearance and improve lubricant access to the cutting edge. Clearly, an appropriate balance between these two conflicting demands must be found, preferably on the basis of the anticipated usage of the tool. The present inventors offer the following ratios as being best for corresponding milling tasks:
The above table shows that when changing the type of milling operation being carried out, for example when a workpiece is to be slotted after its sides have been finish machined, the milling cutter should be changed too. Not only does this situation waste time in the machine shop, but a large stock of different end mills needs to be available if optimum machining results are to be obtained.
Usually when an end mill is in operation the machining force is exerted against one edge of the cutter. The resulting moment is resisted by the tool holder which rigidly grips the cutter shank. Ignoring the fact that the direction of the moment changes continually as the cutter revolves, the cutter can be considered to be stressed as a cantilever. For simplicity, the load can be considered to be a point load. The bending stress on the cantilever is:
W/Z (lxe2x88x92x)
where
W=load
Z=section modulus
l=distance between the tool holder jaw end and the load, and
x=distance between the tool holder jaw end and the point at which stress is to be measured.
Clearly, bending moment increases exponentially as x increases. A constant diameter core, prevalent on prior-art endmills, is therefore not the best way of allocating the limited available space in the design of an end mill.
Two notes need to be added to the above remarks.
1. The actual stress on any beam also includes shear stress, but as shear stress is constant over the considered beam section, it is here ignored.
2. The cutter section, and therefor Z, is not constant, being reduced by the tooth flutes and being full along the shank. Tooth flutes terminate in a slope or curve to prevent stress concentration.
While the matter of stress discussed above is related to avoiding tool breakage, no less important is the requirement to minimize tool deflection, in order to improve accuracy and surface finish and to reduce vibration and noise. A constant diameter core results in more tool deflection than necessary, because the high bending moment close to the tool holder, and also near the ends of the tooth flutes nearest the cutter shank, results in bending of the cutter as a result of machining forces. The resultant cutter deflection at the cutting edge would be reduced if the tool steel comprising the cutter core were distributed in a better manner, that is more metal were allocated to the sections under greater stress, at the expense of the more lightly stressed toothed end of the cutter.
It is therefore one of the objects of the present invention to obviate the disadvantages of prior art end mills and to provide a cutter which offers improved stiffness while yet allowing adequate tooth depth for chip clearance.
The present invention achieves the above object by providing a rotary multi-tooth end milling cutter wherein the total cross-sectional area of the cutter material increases gradually from the cutting end towards the cutter shank.
In a preferred embodiment of the present invention there is provided an end milling cutter wherein in a central cross-section along the length of said cutter the cutter core gradually increases in diameter from the cutting end towards the cutter shank.
In a most preferred embodiment of the present invention there is provided a cutter wherein tooth width increases gradually from the cutting end towards the cutter shank.
Yet further embodiments of the invention will be described hereinafter.
In U.S. Pat. No. 5,452,971 there is described and claimed by Nevills a rotary end cutting tool of great complexity, one of its many special features being xe2x80x9ca tapered residual core area.xe2x80x9d The tool is however a drill, as evidenced by the description and diagrams. In particular, the core area is much smaller than found on end mills and the end of the tool is shaped for penetrating the workpiece by axial pressure to produce or enlarge a hole.
In contradistinction thereto, the present invention relates to end mills which have a substantial core and have side teeth intended for machining flat or curved surfaces.
It will thus be realized that the novel endmill of the present invention serves to provide sufficient stiffness to allow operation on slotting, surface roughing and finishing without needing to change over the cutter. As a result, the cutter produces better quality surfaces, is more resistant to breakage, and less liable to vibration and generating noise. At the same time, adequate tooth depth is provided where most needed, that is near the cutting end of the mill, to allow coolant access to the cutting area and to clear away the generated chips.